Bump up LIBVERSION?

We currently set LIBVERSION to 0:0:0. This means that the
installed share libraries get names like libldap.so.0.0.0. These
numbers don't look nice. We should change them to something else.
At least RedHat have been distributing openldap packages with
version 1.0.0 in libraries.
I propose that we declare that library major version 1 is for
Umich-compatible APIs and that OpenLDAP 2.0, implementing the
new API based on the I-D (where ld is an opaque structure, etc.).
gets a library major version of 2.
In consequence, I propose that we set LIBVERSION to 2:0:0 in
build/top.mk in the HEAD branch, effective immediately.
Comments?
Julio